1. Field of the Invention
The present invention relates to a process for producing N-vinylformamide and, more particularly, to a process for producing N-vinylformamide by generating N-(.alpha.substituted-ethyl)formamide vapor or ethylidenebisformamide vapor by evaporation under reduced pressure and thermally decomposing the vapor at 150.degree.-600.degree. C. in a reactor.
2. Description of the Prior Art
N-vinylformamide is important as a polymerizable monomer for a water-soluble polymer which finds use as a flocculant. According to a known process, it is produced by thermal decomposition in vapor phase of N-(.alpha.-alkoxyethyl)formamide, N-(.alpha.-cyanoethyl)formamide, or ethylidenebisformamide. To be more specific, the thermal decomposition involves the steps of evaporating the abovementioned starting material by heating, subjecting the vapor to thermal decomposition at 300.degree.-600.degree. C. in a packed or unpacked tubular reactor, and cooling the pyrolysis gas. (U.S. Pat. No. 3,914,304 and European Patent Laid-Open No. 184074) However, thermal decomposition in this conventional process tends to deposit tarry or solid resinous substances on the packing or wall of the tubular reactor. In an extreme case, they clog the reactor, hindering stable operation.
In order to avoid the deposition of resinous substances, the present inventors had previously proposed a pyrolysis reactor made up of two stages, the first being an unpacked tubular reactor and the second being a packed tubular reactor. (Japanese Patent Laid-Open No. 181451/991) This reactor functions satisfactorily (without deposition of resinous substances) in an experimental scale so long as it is small enough for the starting material to be heated rapidly; however, it does not in an industrial scale in which the rapid heating of the starting material is difficult. A possible way of overcoming this disadvantage is by extending the residence time of the starting material in the reactor or intensifying the heating of the reactor. However, it is unfavorable for productivity and energy consumption.
In view of the foregoing, the present inventors carried out a series of researches on how to prevent the deposition of resinous substances in the tubular reactor. As the result, it was found that the object is achieved by operating a composite reactor of specific structure at a specific pyrolysis temperature. This finding led to the present invention.